As the LAPC includes an on-board MPU-401, it should be possible to run it as an external unit (with a suitable power supply) and control it via a cheap USB-midi adapter instead of through the ISA bus.

I could be wrong about this but it seems reasonable. I've done it with a SCC-1 card and it works fine.

I like this

http://www.hellotrade.com/cmi-microsystems/product.html

Look at the first item. 7 slot ISA expansion chassis with a PCI slot interface board. Probably expensive as hell, though. Those things usually are.